This invention relates to a process for isomerizing xylenes using a family of related crystalline aluminosilicate zeolites examples of which have been designated UZM-5, UZM-5P and UZM-6. These compositions are structurally different from other known zeolites.
Zeolites are crystalline aluminosilicate compositions which are microporous and which have a three-dimensional oxide framework formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes such as hydrocarbon conversion processes. One particular process which uses zeolites as a catlyst is the isomerization of aromatic compounds, especially the isomerization of C8 aromatic compounds. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure.
Catalysts for isomerization of C8 aromatics ordinarily are classified by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but it normally is converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. A widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. An alternative approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. The former approach commonly results in higher ethylbenzene conversion, thus lowering the quantity of recycle to the para-xylene recovery unit and concomitant processing costs, but the latter approach enhances xylene yield by forming xylenes from ethylbenzene. A catalyst composite and process which enhance conversion according to the latter approach, i.e., achieve ethylbenzene isomerization to xylenes with high conversion, would effect significant improvements in xylene-production economics.
Applicants have synthesized a family of crystalline zeolitic compositions which have unique x-ray diffraction patterns and have the empirical formula on an anhydrous basis in terms of molar ratios of:
Mmn+Rrp+Al(1xe2x88x92x)ExSiyOz
where M is at least one exchangeable cation selected from the group consisting of alkali and alkaline earth metals, xe2x80x9cmxe2x80x9d is the mole ratio of M to (Al+E) and varies from about 0 to about 1.2, R is a nitrogen-containing organic cation selected from the group consisting of quaternary ammonium ions, protonated amines, protonated diamines, protonated alkanolamines, quaternary alkanolammonium ions and diquaternary ammonium ions, and mixtures thereof, xe2x80x9crxe2x80x9d is the mole ratio of R to (Al+E) and has a value of about 0.25 to about 3.0, E is an element selected from the group consisting of Ga, Fe, Cr, In and B, xe2x80x9cxxe2x80x9d is the mole fraction of E and varies from 0 to about 0.5, xe2x80x9cnxe2x80x9d is the weighted average valence of M and has a value of +1 to about +2, xe2x80x9cpxe2x80x9d is the weighted average valence of R and has a value of +1 to about +2, xe2x80x9cyxe2x80x9d is the mole ratio of Si to (Al+E) and varies from about 5 to about 12. and xe2x80x9czxe2x80x9d is the mole ratio of O to (Al+E) and has a value determined by the equation:
z=(mxc2x7n+rxc2x7p+3+4xc2x7y)/2.
Specific members of this family of zeolites are: UZM-5, UZM-5P and UZM-6.
This invention relates to a process for isomerizing aromatic compounds using a new family of zeolites. Accordingly, one embodiment of the invention is a process for the isomerization of a non-equilibrium feed mixture of xylenes and ethylbenzene comprising contacting the feed mixture in the presence of hydrogen in an isomerization zone with a catalyst composite comprising an effective amount of at least one platinum-group metal component and an aluminosilicate crystalline zeolite at isomerization conditions to obtain an isomerized product comprising a higher proportion of p-xylene than in the feed mixture, the zeolite having a composition in the as synthesized form in terms of mole ratios of the elements given by:
Mmn+Rrp+Al(1xe2x88x92x)ExSiyOz
where M is at least one exchangeable cation selected from the group consisting of alkali and alkaline earth metals, xe2x80x9cmxe2x80x9d is the mole ratio of M to (Al+E) and varies from about 0 to about 1.2, R is a nitrogen-containing organic cation selected from the group consisting of protonated amines, protonated diamines, protonated alkanolamines, quaternary ammonium ions, diquaternaryammonium ions, quaternized alkanolamines and mixtures thereof, xe2x80x9crxe2x80x9d is the mole ratio of R to (Al+E) and has a value of about 0.25 to about 3.0, E is at least one element selected from the group consisting of Ga, Fe, Cr, In and B, xe2x80x9cXxe2x80x9d is the mole fraction of E and varies from 0 to 0.5, xe2x80x9cnxe2x80x9d is the weighted average valence of M and has a value of about +1 to about +2, xe2x80x9cpxe2x80x9d is the weighted average valence of R and has a value of +1 to about +2, xe2x80x9cyxe2x80x9d is the mole ratio of Si to (Al+E) and varies from about 5 to about 12 and xe2x80x9czxe2x80x9d is the mole ratio of O to (Al+E) and has a value determined by the equation:
z=(mxc2x7n+rxc2x7p+3+4xc2x7y)/2
the material characterized in that it has at least two x-ray diffraction peaks, one at a d-spacing of 3.9xc2x10.12 xc3x85 and one at 8.6xc2x10.2 xc3x85.
Isomerization conditions comprise a temperature of about 1000 to about 500xc2x0 C., a pressure of about 1 to about 50 atmospheres, a liquid hourly space velocity from about 0.5 to 10 hrxe2x88x921 and a hydrogen-to-hydrocarbon mole ratio from about 0.5:1 to 25:1.
In a particular embodiment, the zeolite catalyst has been designated UZM-5 and has the x-ray diffraction pattern having at least the d-spacings and intensities set forth in Table A:
This and other objects and embodiments will become more apparent after the following detailed description of the invention.
One essential aspect of the process of the invention is a new family of zeolites. Three species have been designated UZM-5, UZM-5P and UZM-6. In its as-synthesized form, these zeolites have a composition on an anhydrous basis that is represented by the formula:
Mmn+Rrp+Al(1xe2x88x92X)ExSiyOz.
M is an exchangeable cation and is selected from the group consisting of alkali and alkaline earth metals. Specific examples of the M cations include but are not limited to lithium, sodium, potassium, cesium, strontium, calcium, magnesium, barium and mixtures thereof. The value of xe2x80x9cmxe2x80x9d which is the mole ratio of M to (Al+E) varies from 0 to 1.2. R is a nitrogen containing organic cation and is selected from the group consisting of protonated amines, protonated diamines, protonated alkanolamines, quaternary ammonium ions, diquaternary ammonium ions, quaternized alkanolammonium ions and mixtures thereof. The value of xe2x80x9crxe2x80x9d which is the mole ratio of R to (Al+E) and varies from about 0.25 to about 3.0. The value of xe2x80x9cnxe2x80x9d which is the weighted average valence of M varies from +1 to about +2. The value of xe2x80x9cpxe2x80x9d, which is the average weighted valence of the organic cation has a value from about +1 to about +2. E is an element which is present in the framework and is selected from the group consisting of gallium, iron, boron, chromium, indium and mixtures thereof. The value of xe2x80x9cxxe2x80x9d which is the mole fraction of E varies from 0 to about 0.5. The ratio of silicon to (Al+E) is represented by xe2x80x9cyxe2x80x9d which varies from about 5 to about 12, while the mole ratio of O to (Al+E) is represented by xe2x80x9czxe2x80x9d and has a value given by the equation:
z=(mxc2x7n+rxc2x7p+3+4xc2x7y)/2.
When M is only one metal, then the weighted average valence is the valence of that one metal, i.e. +1 or +2. However, when more than one M metal is present, the total amount of:
Mmn+=Mm1(n1)++Mm2(n2)++Mm3(n3)++ . . .
and the weighted average valence xe2x80x9cnxe2x80x9d is given by the equation:   n  =                              m          1                ·                  n          1                    +                        m          2                ·                  n          2                    +                        m          3                ·                  n          3                    +      ⋯                      m        1            +              m        2            +                        m                      3            ⁢                          xe2x80x83                                      ⁢        ⋯            
Similarly when only one R organic cation is present, the weighted average valence is the valence of the single R cation, i.e., +1 or +2. When more than one R cation is present, the total amount of R is given by the equation:
Rrp+=Rr1(p1)++Rr2(p2)++Rr3(p3)+
and the weighted average valence xe2x80x9cpxe2x80x9d is given by the equation:   p  =                                          p            1                    ·                      r            1                          +                              p            2                    ·                      r            2                          +                              p            3                    ·                      r            3                          +        ⋯                              r          1                +                  r          2                +                  r          3                +        ⋯              .  
These aluminosilicate zeolites are prepared by a hydrothermal crystallization of a reaction mixture prepared by combining reactive sources of R, aluminum, silicon and optionally E and/or M in aqueous media. Accordingly, the aluminum sources include, but are not limited to, aluminum alkoxides, precipitated alumina, aluminum hydroxide, aluminum salt and aluminum metal. Specific examples of aluminum alkoxides include, but are not limited to aluminum ortho sec-butoxide, and aluminum orthoisopropoxide. Sources of silica include but are not limited to tetraethylorthosilicate, fumed silicas, precipitated silicas and colloidal silica. Sources of the M metals include the halide salts, nitrate salts, acetate salts, and hydroxides of the respective alkali or alkaline earth metals. Sources of the E elements include but are not limited to alkali borates, boric acid, precipitated gallium oxyhydroxide, gallium sulfate, ferric sulfate, ferric chloride, chromium chloride, chromium nitrate, indium chloride and indium nitrate. When R is a quaternary ammonium cation, the sources include without limitation the hydroxide, and halide compounds. Specific examples include without limitation tetramethylammonium hydroxide, tetraethylammonium hydroxide, hexamethonium bromide, tetramethylammonium chloride, methyltriethylammonium hydroxide. R may also be neutral amines, diamines, and alkanolamines. Specific examples are triethanolamine, triethylamine, and N,N,Nxe2x80x2,Nxe2x80x2tetramethyl-1,6-hexanediamine.
The reaction mixture containing reactive sources of the desired components can be described in terms of molar ratios of the oxides by the formula:
aM2/nO:bR2/pO:(1xe2x88x92c)Al2O3: cE2O3:dSiO2:eH2O
where xe2x80x9caxe2x80x9d is the mole ratio of the oxide of M and has a value from 0 to about 2, xe2x80x9cbxe2x80x9d is the mole ratio of the oxide of R and has a value of about 1.5 to about 30, xe2x80x9cdxe2x80x9d is the mole ratio of silica and has a value of about 5 to about 30, xe2x80x9ccxe2x80x9d is the mole ratio of the oxide of E and has a value from 0 to about 0.5, and xe2x80x9cexe2x80x9d is the mole ratio of water and has a value of about 30 to about 6000. The reaction mixture is now reacted at a temperature of about 100xc2x0 C. to about 175xc2x0 C. and preferably from about 140xc2x0 C. to about 160xc2x0 C. for a period of about 12 hours to about 14 days and preferably for a time of about 2 days to about 5 days in a sealed reaction vessel under autogenous pressure. After crystallization is complete, the solid product is isolated from the heterogeneous mixture by means such as filtration or centrifugation, and then washed with de-ionized water and dried in air at ambient temperature up to about 100xc2x0 C.
As synthesized, the zeolites will contain some of the exchangeable or charge balancing cations in its pores. These exchangeable cations can be exchanged for other cations, or in the case of organic cations, they can be removed by heating under controlled conditions. All of these methods are well known in the art.
The crystalline silicoalumino zeolites are characterized by a three-dimensional framework structure of at least SiO2 and AlO2 tetrahedral units. These zeolites are further characterized by their unique x-ray diffraction pattern. The x-ray diffraction pattern has at least two peaks: one peak at a d-spacing of about 3.9xc2x10.12 xc3x85 and one peak at a d-spacing of about 8.6xc2x10.20 xc3x85. To allow for ready reference, the different structure types and compositions of crystalline zeolites have been given arbitrary designation of UZM-h, where xe2x80x9chxe2x80x9d is an integer starting at one and where for example xe2x80x9c1xe2x80x9d represents a framework of structure type xe2x80x9c1xe2x80x9d. That is one or more zeolitic composition with different empirical formulas can have the same structure type xe2x80x9chxe2x80x9d, e.g. xe2x80x9c1xe2x80x9d.
In this respect, the following species can be identified by their x-ray diffraction patterns which have at least the d-spacing and relative intensities set forth in Tables A to C.
The zeolite preferably is mixed with a binder for convenient formation of catalyst particles in a proportion of about 5 to 100 mass % zeolite and 0 to 95 mass-% binder, with the zeolite preferably comprising from about 10 to 90 mass-% of the composite. The binder should preferably be porous, have a surface area of about 5 to about 800 m2/g, and relatively refractory to the conditions utilized in the hydrocarbon conversion process. Non-limiting examples of binders are aluminas, titania, zirconia, zinc oxide, magnesia, boria, silica-alumina, silica-magnesia, chromia-alumina, alumina-boria, silica-zirconia, silica, silica gel, and clays. Preferred binders are amorphous silica and alumina, including gamma-, eta-, and theta-alumina, with gamma- and eta-alumina being especially preferred.
The zeolite with or without a binder can be formed into various shapes such as pills, pellets, extrudates, spheres, etc. Preferred shapes are extrudates and spheres. Extrudates are prepared by conventional means which involves mixing of zeolite either before or after adding metallic components, with the binder and a suitable peptizing agent to form a homogeneous dough or thick paste having the correct moisture content to allow for the formation of extrudates with acceptable integrity to withstand direct calcination. The dough then is extruded through a die to give the shaped extrudate. A multitude of different extrudate shapes are possible, including, but not limited to, cylinders, cloverleaf, dumbbell and symmetrical and asymmetrical polylobates. It is also within the scope of this invention that the extrudates may be further shaped to any desired form, such as spheres, by any means known to the art.
Spheres can be prepared by the well known oil-drop method which is described in U.S. Pat. No 2,620,314 which is incorporated by reference. The method involves dropping a mixture of zeolite, and for example, alumina sol, and gelling agent into an oil bath maintained at elevated temperatures. The droplets of the mixture remain in the oil bath until they set and form hydrogel spheres. The spheres are then continuously withdrawn from the oil bath and typically subjected to specific aging treatments in oil and an ammoniacal solution to further improve their physical characteristics. The resulting aged and gelled particles are then washed and dried at a relatively low temperature of about 50-200xc2x0 C. and subjected to a calcination procedure at a temperature of about 450-700xc2x0 C. for a period of about 1 to about 20 hours. This treatment effects conversion of the hydrogel to the corresponding alumina matrix.
A platinum-group metal, including one or more of platinum, palladium, rhodium, ruthenium, osmium, and iridium, is an essential component of the present catalyst. The preferred platinum-group metal is platinum. The platinum-group metal component may exist within the final catalyst composite as a compound such as an oxide, sulfide, halide, oxysulfide, etc., or as an elemental metal or in combination with one or more other ingredients of the catalyst composite. It is believed that the best results are obtained when substantially all the platinum-group metal component exists in a reduced state. The platinum-group metal component generally comprises from about 0.01 to about 5 mass-% and preferably from about 0.1 to about 2% of the final catalyst composite, calculated on an elemental basis.
The platinum-group metal component may be incorporated into the catalyst composite in any suitable manner. One method of preparing the catalyst involves the utilization of a water-soluble, decomposable compound of a platinum-group metal to impregnate the calcined sieve/binder composite. Alternatively, a platinum-group metal compound may be added at the time of compositing the zeolite and binder. Yet another method of effecting a suitable metal distribution is by compositing the metal component with the binder prior to co-extruding the zeolite and binder. Complexes of platinum-group metals which may be employed according to the above or other known methods include chloroplatinic acid, chloropalladic acid, ammonium chloroplatinate, bromoplatinic acid, platinum trichloride, platinum tetrachloride hydrate, platinum dichlorocarbonyl dichloride, tetramine platinic chloride, dinitrodiaminoplatinum, sodium tetranitroplatinate (II), palladium chloride, palladium nitrate, palladium sulfate, diamminepalladium (II) hydroxide, tetramminepalladium (II) chloride, and the like.
It is within the scope of the present invention that the catalyst composite may contain other metal components known to modify the effect of the platinum-group metal component. Such metal modifiers may include rhenium, tin, germanium, lead, cobalt, nickel, indium, gallium, zinc, uranium, dysprosium, thallium, and mixtures thereof. Catalytically effective amounts of such metal modifiers may be incorporated into the catalyst by any means known in the art to effect a homogeneous or stratified distribution.
The catalyst composite of the present invention may contain a halogen component. The halogen component may be either fluorine, chlorine, bromine or iodine or mixtures thereof, with chlorine being preferred. The halogen component is generally present in a combined state with the inorganic-oxide support. The optional halogen component is preferably well dispersed throughout the catalyst and may comprise from more than 0.2 to about 15 wt. %, calculated on an elemental basis, of the final catalyst. The halogen component may be incorporated in the catalyst composite in any suitable manner, either during the preparation of the inorganic-oxide support or before, while or after other catalytic components are incorporated.
The catalyst composite is dried at a temperature of from about 100xc2x0 to about 320xc2x0 C. for a period of from about 2 to about 24 or more hours and, usually, calcined at a temperature of from 400xc2x0 about 650xc2x0 C. in an air atmosphere for a period of from about 1 to about 10 hours until the metallic compounds present are converted substantially to the oxide form. If desired, the optional halogen component may be adjusted by including a halogen or halogen-containing compound in the air atmosphere.
The resultant calcined composite optimally is subjected to a substantially water-free reduction step to insure a uniform and finely divided dispersion of the optional metallic components. The reduction optionally may be effected in situ. Substantially pure and dry hydrogen (i.e., less than 20 vol. ppm H2O) preferably is used as the reducing agent in this step. The reducing agent contacts the catalyst at conditions, including a temperature of from about 200xc2x0 to about 650xc2x0 C. and for a period of from about 0.5 to about 10 hours, effective to reduce substantially all of the Group VIII metal component to the metallic state. In some cases the resulting reduced catalyst composite may also be beneficially subjected to presulfiding by a method known in the art to incorporate in the catalyst composite from about 0.05 to about 1.0 mass-% sulfur calculated on an elemental basis.
The feedstock to aromatics isomerization comprises isomerizable alkylaromatic hydrocarbons of the general formula C6H(6xe2x88x92n)Rn, where n is an integer from 1 to 5 and R is CH3, C2H5, C3H7, or C4H9, in any combination and including all the isomers thereof to obtain more valuable isomers of the alkylaromatic. Suitable alkylaromatic hydrocarbons include without limitation ortho-xylene, meta-xylene, para-xylene, ethylbenzene, ethyltoluenes, tri-methylbenzenes, di-ethylbenzenes, tri-ethyl-benzenes, methylpropylbenzenes, ethylpropylbenzenes, di-isopropylbenzenes, and mixtures thereof.
Isomerization of a C8-aromatic mixture containing ethylbenzene and xylenes is a particularly preferred application for the zeolites of the invention. Generally such mixture will have an ethylbenzene content in the approximate range of 5 to 50 mass-%, an ortho-xylene content in the approximate range of 0 to 35 mass-%, a meta-xylene content in the approximate range of 20 to 95 mass-% and a para-xylene content in the approximate range of 0 to 15 mass-%. It is preferred that the aforementioned C8 aromatics comprise a non-equilibrium mixture, i.e., at least one C8-aromatic isomer is present in a concentration that differs substantially (defined herein as a difference of at least 5 mass-% of the total C8 aromatics) from the thermodynamic equilibrium concentration of that isomer at isomerization conditions. Usually the non-equilibrium mixture is prepared by removal of para- and/or ortho-xylene from a fresh C8 aromatic mixture obtained from an aromatics-production process, and preferably the non-equilibrium mixture contains less than 5 mass-% para-xylene.
The alkylaromatic hydrocarbons may be utilized in the present invention as found in appropriate fractions from various refinery petroleum streams, e.g., as individual components or as certain boiling-range fractions obtained by the selective fractionation and distillation of catalytically cracked or reformed hydrocarbons. The isomerizable aromatic hydrocarbons need not be concentrated; the process of this invention allows the isomerization of alkylaromatic-containing streams such as catalytic reformate with or without subsequent aromatics extraction to produce specified xylene isomers and particularly to produce para-xylene. A C8-aromatics feed to the present process may contain nonaromatic hydrocarbons, i.e., naphthenes and paraffins, in an amount up to 30 mass-%. Preferably the isomerizable hydrocarbons consist essentially of aromatics, however, to ensure pure products from downstream recovery processes.
According to the process of the present invention, an alkylaromatic hydrocarbon feed mixture, preferably in admixture with hydrogen, is contacted with a catalyst of the type hereinafter described in an alkylaromatic hydrocarbon isomerization zone. Contacting may be effected using the catalyst in a fixed-bed system, a moving-bed system, a fluidized-bed system, or in a batch-type operation. In view of the danger of attrition loss of the valuable catalyst and of the simpler operation, it is preferred to use a fixed-bed system. In this system, a hydrogen-rich gas and the feed mixture are preheated by suitable heating means to the desired reaction temperature and then passed into an isomerization zone containing a fixed bed of catalyst. The conversion zone may be one or more separate reactors with suitable means therebetween to ensure that the desired isomerization temperature is maintained at the entrance to each zone. The reactants may be contacted with the catalyst bed in either upward-, downward-, or radial-flow fashion, and the reactants may be in the liquid phase, a mixed liquid-vapor phase, or a vapor phase when contacted with the catalyst.
The alkylaromatic feed mixture, preferably a non-equilibrium mixture of C8 aromatics, is contacted with the isomerization catalyst at suitable alkylaromaticisomerization conditions. Such conditions comprise a temperature ranging from about 0xc2x0 to 600xc2x0 C. or more, and preferably is in the range of from about 100xc2x0 to 500xc2x0 C. The pressure generally is from about 1 to 100 atmospheres absolute, preferably less than about 50 atmospheres. Sufficient catalyst is contained in the isomerization zone to provide a liquid hourly space velocity with respect to the hydrocarbon feed mixture of from about 0.1 to 30 hrxe2x88x921, and preferably 0.5 to 10 hrxe2x88x921. The hydrocarbon feed mixture optimally is reacted in admixture with hydrogen at a hydrogen/hydrocarbon mole ratio of about 0.5:1 to about 25:1 or more. Other inert diluents such as nitrogen, argon and light hydrocarbons may be present.
The reaction proceeds via the mechanism, described hereinabove, of isomerizing xylenes while reacting ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes. The yield of xylenes in the product thus is enhanced by forming xylenes from ethylbenzene. The loss of C8 aromatics through the reaction thus is low: typically less than about 4 mass-% per pass of C8 aromatics in the feed to the reactor, preferably about 3 mass-% or less, and most preferably no more than about 2.5 mass-%.
The particular scheme employed to recover an isomerized product from the effluent of the reactors of the isomerization zone is not deemed to be critical to the instant invention, and any effective recovery scheme known in the art may be used. Typically, the reactor effluent will be condensed and the hydrogen and light-hydrocarbon components removed therefrom by flash separation. The condensed liquid product then is fractionated to remove light and/or heavy byproducts and obtain the isomerized product. In some instances, certain product species such as ortho-xylene may be recovered from the isomerized product by selective fractionation. The product from isomerization of C8 aromatics usually is processed to selectively recover the para-xylene isomer, optionally by crystallization. Selective adsorption is preferred using crystalline aluminosilicates according to U.S. Pat. No. 3,201,491. Improvements and alternatives within the preferred adsorption recovery process are described in U.S. Pat. Nos. 3,626,020, 3,696,107, 4,039,599, 4,184,943, 4,381,419 and 4,402,832, incorporated herein by reference thereto.
In a separation/isomerization process combination relating to the processing of an ethylbenzene/xylene mixture, a fresh C8-aromatics feed is combined with isomerized product comprising C8 aromatics and naphthenes from the isomerization reaction zone and fed to a para-xylene separation zone; the para-xylene-depleted stream comprising a non-equilibrium mixture of C8 aromatics is fed to the isomerization reaction zone, where the C8-aromatic isomers are isomerized to near-equilibrium levels to obtain the isomerized product. In this process scheme non-recovered C8-aromatic isomers preferably are recycled to extinction until they are either converted to para-xylene or lost due to side-reactions. Ortho-xylene separation, preferably by fractionation, also may be effected on the fresh C8-aromatic feed or isomerized product, or both in combination, prior to para-xylene separation.